This invention relates generally to impact detectors and more particularly to an impact detector for detecting impact of a moving object, such as a vehicle, with an obstacle.
Impact detectors for detecting impact of a moving object, such as a vehicle, with an obstacle, are known. Generally such detectors include a movable seismic mass which is located in a static or nondisplaced position by some type of motion resistant force, such as an elastic spring, a magnet, an inclined surface, or various combinations of such forces. The seismic mass closes switch contacts when the impact detector is subjected to an impulsive force of sufficient magnitude and duration which is applied to the seismic mass in a direction generally opposite the direction of movement of the seismic mass. The impulsive force must also be of sufficient magnitude and duration to overcome any damping force applied to the seismic mass during movement, such as by viscous dampers, electromagnetic dampers, or otherwise. The impact detector is mounted to the moving object, such as a vehicle, and the switch contacts are connected to circuitry which uses the impact detection information for various purposes, such as to actuate an inflatable occupant restraint system.
The impact detector of this invention is of this general type and includes: a movable seismic mass located in a static or nondisplaced position by a motion resistant force; viscous fluid damping; and, switch contacts which are closed when the seismic mass moves to its displaced position when the impact detector is subjected to an impulsive force of sufficient magnitude and duration and in the appropriate direction. Closure of the switch contacts can actuate a vehicle occupant protection device, such as an inflatable occupant restraint system.
In its preferred embodiment, the impact detector includes a center chip which is a micromachined silicon wafer having an integral seismic mass, a perimeter ring surrounding the seismic mass, and a plurality of integral beams interconnecting the seismic mass and the perimeter ring and applying a tensile force to the seismic mass locating the seismic mass in a static or nondisplaced position and resisting movement of the seismic mass toward a displaced position. A back plate is sandwiched to one side of the center chip and includes a number of switch contacts which are spaced from switch contacts on the seismic mass and are part of impact detection circuitry for actuating an occupant protection device, such as an inflatable occupant restraint system. A cover plate is sandwiched to the other side of the center chip. An electrostatic voltage is applied between the seismic mass and the back plate. Normally, the electrostatic attractive force is insufficient to overcome the motion resistant tensile force applied to the seismic mass by the beams so that the seismic mass is maintained in its static or nondisplaced position relative to the back plate. When an impulsive force of sufficient magnitude and duration and in the appropriate duration, generally opposite the direction of movement of the seismic mass toward displaced position, is applied to the impact detector, the electrostatic attractive force coupled with the inertial reaction of the seismic mass overcomes the tensile force and moves the seismic mass to the displaced or operative position wherein switch contacts on the seismic mass close to the switch contacts on the back plate and actuate the impact detection circuitry. Once the switch contacts close, the electrostatic attractive force maintains the switch contacts latched to each other. The inertial reaction of the seismic mass and the electrostatic attractive force must be sufficient to overcome the tensile force and the viscous damping force resulting from displacement of the air or other viscous fluid between the seismic mass and the back plate as the seismic mass moves to the displaced position.
The switch contacts remain latched to each other until such time as the electrostatic voltage is reduced and the tensile force is sufficient to overcome the reduced electrostatic attractive force and return the seismic mass to its static or nondisplaced position. This obviates the need for a timing circuit as part of the circuitry for the occupant protection device. The electrostatic voltage can also be increased when the seismic mass is in its nondisplayed position to increase the electrostatic attractive force to a level where it overcomes the tensile force and moves the seismic mass to the displaced position to close the switch contacts without any impulsive force being applied to the impact detector. This provides the impact detector with an in situ check capability permitting checking of the impact detector and the circuitry when desired or at predetermined intervals.
A primary feature of the impact detector of this invention is that it includes a seismic mass located in a static or nondisplaced position by tensile force applied to the seismic mass and balancing an electrostatic attractive force biasing the seismic mass to a displaced position. Another feature is that the electrostatic attractive force results from and electrostatic voltage applied between the seismic mass and a switch contact support spaced therefrom, with the space between the seismic mass and switch contact support containing a viscous damping medium. A further feature is that the seismic mass carries first switch contacts which are closed and latched to second switch contacts on the switch contact support when the impact detector is subjected to an impulsive force of sufficient magnitude and duration, with the resultant inertial reaction of the seismic mass coupled to the electrostatic attractive force being sufficient to overcome the tensile force and any damping forces resulting from displacement by the seismic mass of the viscous damping medium separating the seismic mass and the switch contact support. Yet another feature is that the switch contacts remain latched until the electrostatic voltage is reduced to a level where the electrostatic attractive force is overcome by the tensile force. Yet a further feature is that the electrostatic voltage can be increased to increase the electrostatic attractive force to a level where it overcomes the tensile force and moves the seismic mass from its static or nondisplaced position to its displaced position in order to provide an in situ check of the operability of the impact detector and the circuitry connecting the impact detector with an occupant protection device. Still another feature is that the seismic mass is part of a micromachined silicon wafer and the tensile force locating the seismic mass in a static or nondisplaced position is provided by a plurality of beams supporting the seismic mass within a perimeter ring of the silicon wafer.